The present invention relates to an apparatus and a method for monitoring body organs, and more particularly to an apparatus and a method for measuring the oxygen content of a body organ (that is, oxygen partial pressure in the body organ) non-invasively.
In a living body, there is always carried out a process that adenosine triphosphate (ATP) serving as an energy source is produced by consuming oxygen and glucose. Accordingly, in order to diagnose the metabolic state of the living body, it is very important to know how oxygen and glucose are distributed in the living body, non-invasively. Recently, various methods of measuring the oxygen content of a living body optically have been tried, and a part of the methods has been put in practical use. The above methods utilize a fact that the optical characteristics of hemoglobin included in a red blood cell for carrying oxygen to the whole of a living body, the optical characteristics of cytochrome playing an important role in a process for producing adenosine triphosphate in the mitchondrion included in a cell, and the optical characteristics of myoglobin for storing oxygen in a muscle, vary depending upon the state of oxidation, that is, their optical characteristics at the oxygenated state are different from those at the deoxygenated state. Specifically, a method of determining the oxygen content from a change in light absorption spectrum is most frequently used. According to this method, as disclosed in U.S. Pat. No. 4,281,645 in detail, light is introduced into a living body, the light absorption coefficient of an oxygen content indicator substance which is to be examined, is measured, the oxygen saturation of the indicator substance (that is, how much oxygen is introduced into the indicator substance) is determined from the light absorption coefficient of the indicator substance, an oxygen content is determined from the oxygen saturation on the basis of a predetermined relation between the oxygen content and the oxygen saturation. That is, the oxygen partial pressure in the living body can be determined. In the greater part of conventional methods, the oxygen partial pressure is determined on the basis of the above principle. Recently, the imaging of the distribution of oxygen partial pressure has been tried (refer to, for example, Proc Natl Acad Sci U.S.A., Vol. 85, Jul., 1988, pages 4971 to 4975).
However, a curve indicating the relation between the oxygen saturation and the oxygen partial pressure (that is, oxygen dissociation curve) is shifted, depending upon the acidity (pH) and temperature T of a portion to be examined. Accordingly, the oxygen partial pressure determined only from the oxygen saturation data (which is obtained from the light absorption coefficient) does not indicate a correct oxygen partial pressure, but contains an error. When the above oxygen partial pressure is used for diagnosing whether a living body is normal or abnormal, the diagnosis may be erroneous.
As mentioned above, the conventional technology does not take into consideration the shift of oxygen dissociation curve due to the acidity (pH) and temperature of that portion of a person which is to be examined. Thus, the conventional technology cannot determine a correct oxygen partial pressure nor image the distribution of correct oxygen partial pressure.